editor's blog
Subscribe Now

From Relative to Absolute Altitude

GPS is notoriously inaccurate when it comes to vertical positioning. And it disappears entirely inside buildings. So pressure sensors are used to help calculate vertical positioning.

The thing is, a pressure sensor decides your altitude based on the pressure of the air, so it must be comparing it to some baseline. The problem with that is that there is no firm baseline pressure: weather, as we all know, affects the air pressure.

That means that pressure is, first of all, a moving target. Secondly, we can never really know our absolute altitude, only relative.

I posed these questions in a conversation with the Bosch Sensortec team at the MEMS Executive Congress where they were discussing the upcoming release of their new pressure sensors. They talk about being able to handle absolute altitude, so the obvious question is, what about the weather?

There are two pieces to the answer. The first deals with the fact that the baseline pressure isn’t constant. However, compared to pressure changes due to typical motion, the weather pressure changes extremely slowly. (If it’s changing so fast that it could be confused with you moving around, then navigation error is the least of your problems.) From a signal standpoint, the pressure changes of interest can be extracted with a high-pass filter, at least conceptually. More simply, you can think of it as a differential-mode measurement, with actual weather pressure being a common-mode error that’s subtracted out.

That allows you to get a reasonably accurate measure of relative altitude, but what about absolute altitude? Now you need to compare yourself to a sea-level baseline, and that baseline does depend on the weather. Well, there’s no magic available on this. The Bosch Sensortec software can get the data necessary to correct for the current sea-level pressure from the internet. Given that external sanity check, a pressure sensor can provide absolute altitude.

There are a couple other “faster-twitch” effects that can confuse pressure interpretation. The first is simply the fact that some buildings or rooms may have higher or lower air pressure based on the air conditioning or intentional implementation of things like positive pressure for a clean room. Even just opening a door can send a pressure surge. These effects won’t be eliminated or “de-convoluted” in the same way that weather impacts can be. Instead, the pressure data must be fused with other data to decide whether the pressure change reflects a change in altitude. Specifically, if an inertial sensor shows no vertical motion, then the pressure change can be “ignored” (although now it becomes the new baseline).

Pressure measurements also depend on temperature: a local temperature change can register as a pressure change when in fact the pressure didn’t change. Good temperature compensation is required (which is essentially data fusion between a thermometer and a pressure sensor); a pressure sensor less affected by temperature (as is claimed by Bosch Sensortec for their new BMP280) can also help.

Leave a Reply

featured blogs
Apr 9, 2021
You probably already know what ISO 26262 is. If you don't, then you can find out in several previous posts: "The Safest Train Is One that Never Leaves the Station" History of ISO 26262... [[ Click on the title to access the full blog on the Cadence Community s...
Apr 8, 2021
We all know the widespread havoc that Covid-19 wreaked in 2020. While the electronics industry in general, and connectors in particular, took an initial hit, the industry rebounded in the second half of 2020 and is rolling into 2021. Travel came to an almost stand-still in 20...
Apr 7, 2021
We explore how EDA tools enable hyper-convergent IC designs, supporting the PPA and yield targets required by advanced 3DICs and SoCs used in AI and HPC. The post Why Hyper-Convergent Chip Designs Call for a New Approach to Circuit Simulation appeared first on From Silicon T...
Apr 5, 2021
Back in November 2019, just a few short months before we all began an enforced… The post Collaboration and innovation thrive on diversity appeared first on Design with Calibre....

featured video

Learn the basics of Hall Effect sensors

Sponsored by Texas Instruments

This video introduces Hall Effect, permanent magnets and various magnetic properties. It'll walk through the benefits of Hall Effect sensors, how Hall ICs compare to discrete Hall elements and the different types of Hall Effect sensors.

Click here for more information

featured paper

Understanding the Foundations of Quiescent Current in Linear Power Systems

Sponsored by Texas Instruments

Minimizing power consumption is an important design consideration, especially in battery-powered systems that utilize linear regulators or low-dropout regulators (LDOs). Read this new whitepaper to learn the fundamentals of IQ in linear-power systems, how to predict behavior in dropout conditions, and maintain minimal disturbance during the load transient response.

Click here to download the whitepaper

featured chalk talk

Fundamentals of ESD/TVS Protection

Sponsored by Mouser Electronics and Nexperia

ESD protection is a critical, and often overlooked design consideration in many of today’s systems. There is a wide variety of solutions available for ESD protection, and choosing the right one for your design can be a daunting and confusing task. In this episode of Chalk Talk, Amelia Dalton chats with Tom Wolf of Nexperia about choosing the right ESD protection for your next design.

Click here for more information about Nexperia PCMFxUSB3B/C - CMF EMI filters with ESD Protection